Having been through that nomination/appointment process myself, it would be very difficult to catch someone like him who is smart and knows how to keep controversial opinions under wraps. He’d simply avoid mentioning those ideas, and you wouldn’t know. Very difficult to get inside someone’s actual head, and the process isn’t as rigorous as a TS/SCI investigation would be.
I’m sure those ideas were in his head before his 18th birthday. The article suggests he became radicalized after being deployed to Afghanistan.
Free speech allows him to speak his mind. The ‘system’ is too big to stay on top of these things.